Conventionally, a technique has been known which supplies an ultracold liquid such as liquid nitrogen to a vacuum insulated pipe containing a superconducting cable or the like to hold the superconducting cable or the like in an ultracold state. A liquid supply (circulation) system for an ultracold liquid constantly supplies an ultracold liquid into a vacuum insulated pipe so as to cause a to-be-cooled device in which a superconducting cable is provided in the vacuum insulated pipe to hold the superconducting cable in a superconductive state.
A conventional superconducting cable has a short length and, as the discharge pressure required by a liquid supply system, a comparatively low discharge pressure relative to a flow rate is sufficient. Accordingly, as a pump mechanism, a centrifugal pump is representatively used in most cases. However, since a future superconducting cable may have a long length of several kilometers and a place where the superconducting cable is laid may have a level difference, the discharge pressure required by a liquid supply system relative to a flow rate is higher than required conventionally. In a liquid supply system using a centrifugal pump mechanism, the discharge pressure of a pump is low. Therefore, to transfer a liquid over a long distance using only the liquid supply system, it is necessary to, e.g., arrange a plurality of pumps along the cable and maintain the discharge pressure, resulting in higher cost. In addition, when the topography of the place where the cable is laid has a level difference, the pump discharge pressure becomes insufficient so that the laying of the cable is limited.
As another liquid supply system, a positive-displacement bellows circulator as shown in each of FIGS. 5(a) to 5(c) is known (See PTL 1). However, since a conventional positive-displacement bellows circulator has a structure in which an internal pressure is exerted on a bellows, it is comparatively difficult to provide a higher-pressure positive-displacement bellows circulator. In addition, when a high discharge pressure is exerted as an internal pressure on a bellows, the bellows may buckle. Moreover, since a configuration is used in which a vacuum insulated vessel is filled with an ultracold liquid and the positive-displacement bellows circulator is inserted and immersed therein, heat in-leak results from heat transfer via the supporting member of the positive-displacement bellows circulator and also from heat transfer via the wall surface of the vacuum insulated vessel.